Does the CR of a monster take into account anything it can summon? I'm specifically thinking of the Summon Solar Dragon feature of the Astral Elf Aristocrat.  It is a cr8 that can summon another cr8.  When building an encounter with Astral Elves, do I add the dragon to the total XP of the encounter?

Thanks in advance!

I honestly always thought summons wouldn't give XP, but apparently they do. it says in the Monster Manual, page 9: Unless something tells you otherwise, a monster summoned by a spell or other magical ability is worth the XP noted in its stat block.

So yeah if your party kills the Astral Elf Aristocrat and the Solar Dragon they would get XP for both! Although killing the Astral Elf first will dismiss the Solar Dragon so it wouldn't be killed. Rather you give XP in that case is all up to you.

Monster Manual also says: The number of experience points (XP) a monster is worth is based on its challenge rating. Typically, XP is awarded for defeating the monster, although the DM may also award XP for neutralizing the threat posed by the monster in some other manner.

Causing the Solar Dragon to despawn kind of is neutralizing the treat in a way I'd say. Not to mention of course that more XP = Happy Players.

Hope this helped! :)  

I'm not worried about awarding XP.

i'm more worried about planning a challenging encounter.  2 cr 8s is a lot more to plan for than just the 1.

-M

Oh I see my bad, well the CR always takes all the creatures abilities into consideration, so the fact that it can summon an extra CR 8 makes it a CR 8 creature.

Does that make sense? Not at all but CR isn't really the most accurate thing D&D offers unfortunately. Maybe if things start looking south during the combat you could point out to the players that by defeating the summoner you get rid of the summon. 

But yeah the difference between there being a CR8 dragon and not is indeed pretty big. So Perhaps you can predetermine if the 50% goes off or not so you can balance around it. 

Hope your party makes it!